Tom puzzles about WIG Boats

I've been interested in WIG boats for a decade or so. The low-energy soaring of pelicans near waves, the ground effect on a frisbee floating instead of landing after gliding down near to the ground, and my ground-school teacher explaining ground effect to cousin Rosanna and me in 2001, have inspired me to think about this subject. The Russians call it the "screen", or "ekran", as though there is a kind of screen along the surface of the ground or water, that lets you through and partly doesn't let you through, as you're landing.

Even before that, my friend Norbert Wu's father, who was an aerodynamics professor at Georgia Tech, had explained to me once about vortex generation under bird wings, and how commercial aviation is trying to figure out how to use that tremendous lifting power.

Driving home from ground school with Rosanna, I started thinking of designs of a boat with wings.

My first thoughts had water propellers, and a canard wing in front.

It seemed to me that a small wing out front, a canard, could generate a downdraft below the main wing, producing greater pressure there and greater lift. (See WeberWig1 photos.)

And I thought, a water prop might be more efficient because it's pushing against an incompressible and dense material, water, which ought to be better for mechanical energy transfer than pushing against a compressible light material, air. Also WIG boats have a limited safe velocity range, so it might be an advantage if the power mechanism disengages when the boat flies up too high, and the prop comes out of the water. But apparently air props can be efficient too. Finally, the efficiency of a high-speed boat prop depends on the blade being half in and half out of the water, a very very precise elevation requirement, which even medium waves will prevent. So I'm open to air propellers too.

Sponsons in front

Kevin Curran's Lehigh University thesis on aerodynamics of high-speed sponsons influenced my thinking: the front left and right water-contacting corners of a WIG boat should be a bit like Curran's Sponson A, that is, aerodynamically neutral in varying angles of attack, but hydrodynamically efficient, with a step to reduce wetted area at speed.

Three points of contact

A balanced WIG boat should work well as a high-speed boat also, with stability and efficiency operating in full water contact. For high speed, I like wide-set front sponsons and rear drive. (Check out this example, I'll call it the 2015 Edderitz boat.)

RC Models to emulate

I love this guy's work: ground effect vehicle over snow and planing water vehicle. You might say his experiments are primitive but I say they are brilliant and moving things in the right direction. To fix the one-wing-in-contact-with-the-water issue, I say, turn the pontoon into a knife edge so it has minimum drag and go ahead and let it contact the water in normal operation I like it that his prop is above the wing because higher speed air is lower pressure so more lift up there. These designers with PAR (power assisted reinjection of air under the wing) are creating the wrong effect..
His snow-sled flies nicely in ground effect with estimated dimensions 1: rear vane height and length; 3 sled length; 2.5 sled width; 0.8: propeller axis elevation above sled frame; 2: distance from sled tail to delta wing front root; .75: distance from sled tail to delta wing front outside corner. 1.25 width of delta wing, each side. Rear pitch-control surface: full width, 0.5 length. 2.5/12 slope of propeller axis. He has end pontoons, full length,

Materials to build experimental systems

  • For hand-made RC prototypes: Blue or pink insulation foam. EPP foam. 6mm Devron foam. 1/2" wood dowels (square). Grayson Hobby Super mega Jet electric motor.80Amp ESC. Nanotech 3S lipopack 2200. or 1300mAh 3 cell battery. Glued receiver for waterproofing. Servos to turn control surfaces (rudder). Blue Wonder motors. Radio controller: DSM2 DX7 Model Match[TM] technology. 3S brushless motor with watercooling for high speed boats.
  • For full size systems: aluminum boat building technology.

Wing In Ground effect boats: design considerations

  • Names: Naming is a struggle for this category of most fascinating vehicles. It has the worst names: ekranoplan or plane screen in Russian, WIG, AGEC, WISES, these names suck. I don't have a solution, though I don't mind airboat or wingboat. Anyhow W-IN-G sounds more like WING than WIG to me. Whatever, just use all the names in your web pages so everyone can find it, whatever you want to call it. I hate it that we're stuck with WIG Boat.
  • Center of lift variation. So the basic thing is that if your ground effect is air stagnation under the wing, then the lift comes primarily at the trailing edge of the wing where all that air is finally maximally crushed under the trailing edge, and that pushes the vehicle up. The air at the front part isn't compressed yet, so it doesn't push upward that much. So stagnation effect lift is located rearward. Then as soon as the vehicle rises out of stagnation ground effect, the air isn't nearly as compressed at the trailing edge, and the whole wing contributes more equally to the total effect, irrespective of front or back, so the center of lift moves forward. Therefore once the vehicle catches some lift to get 1/4 or more of a wingspan up, or especially if in the back it gets up even just a little, then the relative compression at the trailing edge drops off dramatically (as compared with a thin edge of air under a 5-percent elevation-to-span ratio scenario, for example), and the center of lift distributes forward to the center of the wing, and because you were balanced on the rear-edge center of lift, now suddenly the whole thing flips up and backwards and you get this and this and this. That's what I'm saying. This is a problem. So what's the solution? Don't use stagnation (chord-dominated) ground effect, use span-dominated ground effect in your design. If you are just designing a fast boat, make sponsons in front (or get this = face-plant crash with sponsons not in front) and make them aerodynamically neutral (or get this - backflip crash with sponsons in front but not aerodynamically neutral).
  • Notes on Edwin Van Opstal:
    • Comparing the two sources of ground effect, span-dominated versus chord-dominated, the main effect is span-dominated. Van Opstal's graph shows that at a height of 5% of wingspan, the drag is reduced by to a 30% fraction compared with free flight. Whereas at a height of 5% of the chord, the drag reduces only from 1.1 to 0.8 as compared with free flight. 5% of the CHORD! It would require a chord longer than the average wingspan to get far less than half the effect, if my reasoning is correct. Therefore ground effect is hugely dominated by the wing span effect. And therefore making a long-chord wing, which many designs are based on including Lippisch and Jorg, is a mistake; whereas a short-chord, longer span wing, is the way to maximize ground effect. Witness low-soaring birds such as seagulls and pelicans: Long narrow wings. If the Alexeev ekranoplan designs have short wings, it's because the lift is so great they don't need more.
  • The much-higher power requirement for take-off can be remedied by externally-applied acceleration: a slingshot launch method. Imagine the WIG-boat can putt-putt along the water, or fly above it, but can't make the transition without help. That's safe but if not necessarily convenient, because if you fall out of the sky you can still putt-putt along to wherever you need to go. Eventually.
  • If wave height is normally distributed, then occasional freak waves are inevitable, and the design of WIG boats should allow for non-catastrophic water contact during normal flight. This means knifing sponsons in the front corners where the bounce off the water is pretty soft.
  • I started with a water prop concept when I first started imagining these. Google the Kawasaki KAZ-2
  • Water contact is a drag. Well, based on water density = 1728x steam density, I think a vehicle that transitions smoothly between water phase and air phase lift, propulsion, and vaning, should have roughly 1000x larger air surfaces.
    • Imagine two motors, one for an air prop and one for a water prop; the air prop pushes pi*Ra*Ra*Da in one rotation, which should be 1000x more volume than the water prop's pi*Rw*Rw*Dw. So if both screw forward through the same height of cylinder of the medium in one propeller rotation, then Dw = Da. Equating and cancelling, we have Rw*Rw*1000 = Ra*Ra or Rw*32=Ra. Thus if a hovercraft pusher prop has radius 32" (diameter 64"), then the equivalent water prop should have radius 1" (diameter 2"). I think I must be missing some factor, perhaps the Dw=Da assumption is wrong, or the normal RPMs are lower in the water, because this result seems a little too far in the direction of my argument that only a small water prop is required to get plenty of propulsion, compared with an air prop.
    • Similarly an air wing should pass over and push downward a volume of 1000x the water that a hydrofoil should effect. If the same square area relationship applies, then an air wing of say 3.2'x10' would be the equivalent of a hydrofoil lifting surface of 6-3/4" x 21-1/4". I don't trust my reasoning here at all. But it tends in the right direction, that a very small lifting surface in the water would be equivalent to a reasonably large air wing.
    • Finally, considering vaning effects as by a rudder (in WIGs we should pretty automatically have attitude stability or we have a bad design), here a tiny rudder in the water has the effect of an enormous rudder in the air. To get the 32 square feet of surface for an air rudder shaped as a crude approximation like an isosceles right triangle, we need 8' legs on the triangle! (8 x 8 = 64 = area of a square, the isosceles right triangle is half that square).
    • I think you know where I'm going here. Consider a vehicle that handles equally with air propulsion, lift, and control surfaces and with water propulsion, lift, and control surfaces. Two engines, two propellers, one for each medium. Air wings and hydrofoils, both. And an air rudder like the giant Jorg tail (see here) AND a water rudder and front-corner sponsons. Effects in each medium should be coordinated, but can help each other during transitions, for example, the vehicle will be lifted by flotation then hydroplaning then aeroplaning with a weighted mixture of effects adding to the total results for lift, and similarly for propulsion and control.
    • I'm suggesting front right and left corner sponsons, aerodynamically neutral, but over quite small and thin, perhaps knife-like vertical or curving hydrofoils which can impact the water during flight while creating only minimal drag, up to a certain wave height. Their water displacement volume should remain small up to fairly deep penetrations as through wave tops or to stabilize flight when rolling onto one side or the other, and get larger in displacement only when the sponson is operating as an actual float.
    • I read or saw somewhere that compromise designs lose on both fronts, but I guess I have to see this for myself. Evidently a specific wave height and separation distribution should be the basis of design, and should be practically enforced as hard limits on actual operation, particularly as to take-off.
    • I have in mind some mechanisms for modifying wing/body parameters.
      • One is a "shoulder blade", a mechanism for raising the wing from low-wing (in WIG flight) to high-wing (in flotation). I don't like the wing stuck in the water during taxiing or docking, it should be high. Some rotating truss design should be able to lower the wing once airborn (to reduce wave battering on the body at a given wing elevation above the surface).
      • Another is a parking mechanism, rotating a single-span wing back, or folding two spans on each side against each other like a bird. The bird model is attractive if probably unrealistic, but should be understood: after landing they fuss around and fold the wings under up and back until finally at a low-energy rest position. Upon takeoff, they sweep backward generating a backward-rolling horizontal-axis vortex, followed by a forward sweep making use of the added lift produced by flying over a vortex that is rolling under your wing. This is Dr. Wu's point.
      • Many such adjustments such as folding multi-span wings could require quite manipulable wing shapes, and in turn the feather concept has its value, as feathers overlappingly form a conjoint surface of manipulable thickness and shape, each very light and controlled at the attachment end by shrugs and stretches in coordination with the whole group to form the wingform of current utility, depending on landing, lifting off, soaring, etc. Of course bats fly happily without feathers, depending on a skin stretch factor that is an alternative for us also in this design space.
    • I don't much like the jets blowing air under the wings for initial liftoff. Alexeev spent a lot of time on this one.
      • Doesn't fast air above slow air below represent the basic idea of lift on a curved-top airfoil? Bernoulli, anyone? Then why blow fast air underneath? It can only work in stagnation, when the rear edge of the wing is approximately underwater, then you're blowing up a very leaky balloon with a jet engine, sure it'll blow up a bit, but it'll spray water everywhere like crazy and it'll barely work. Mostly blowing between wing and water amounts to sucking the wing down to be close to the water. Delifting, not lifting. Lifting only at the barest beginning, getting the wings just out of the water. Maybe that's an argument for super low wings, though, as you see Alexeev's planes were pretty flat bottomed. But I see the old two-balloons experiment: blow between them (fast air) and the balloons bang together, bang bang bang bang, a little of the pressing apart happens, but mostly a lot of pulling them together happens. And then it's a repetitive banging thing. Which we don't want, neither the pulling two surfaces together with fast air rushing between them, nor the repetitive banging. So for me for now, let's scratch the PAR (power assisted ram) idea, okay? You wiser people, please persuade me otherwise. Meanwhile, let's forget it.
      • Why not use normal propulsion but have the wings already out of the water, and have a hydrodynamic design that allows smooth acceleration up to the point of liftoff. PAR to get the wings out of the water is then unnecessary. Me, I say have a high wing on a shoulder blade, have a water prop pushing at low speeds and jointly with air propulsion during liftoff acceleration, then let it spin out and crank up out of the way after liftoff.
      • You might even want to force the beast to stay in water contact until past the point of minimum liftoff speed, in order to make use of the water propulsion up to a higher speed, then let it pop up out of the water, letting the water propulsion system shut down when there is enough power in the air propulsion system to keep it flying. What would that take, eh?
    • Consider as a goal for design a WIG boat with the abilities of the 2015 Edderitz boat, which keeps stably close to the surface. The Edderitz boat does a great job balancing the two forces of air pushing it down to the surface and water keeping it above the surface. A similar balance, even more difficult to achieve, would be between the non-lift at a higher elevation due to being out of ground effect, and the lift at a lower elevation due to ground effect. For each velocity there is some optimum, and if stability can be achieved, more than stability but a set of parameters where given the velocity then the elevation is specified and there are strong forces pushing both up and down to keep the vehicle at that elevation. That's the goal. The Edderitz boat at 1/8 to 1/2 wingspan elevation. How can we achieve that? Fly by wire? "Pre-tensioned" hydraulic actuators following a balance point? What are the forces, can we quantify them and design to float at the right points? That's what I'd like to see.

Copyright © 2000-2017, Thomas C. Veatch. All rights reserved.
Modified: June 24, 2017